Methods for manufacture of valve flaps for cardiac valve prostheses

ABSTRACT

Sheets of incompletely fixed biological tissue comprising portions for defining valve flaps of a cardiac valve prosthesis are mounted on forming means which can separate in a substantially fluid-tight manner the opposite faces of such portions. These opposite faces are subjected to a fluid pressure difference which produces deformation of the portions towards a conformation substantially identical with the conformation of the valve flaps when mounted in the prosthesis. The biological tissue is finally fixed while the portions defining the valve flaps of the prosthesis are maintained in the said conformation. The sheets of biological tissue are then separated from the forming means to be subsequently mounted in the frame of the prosthesis.

The present invention relates to methods for the manufacture of valveflaps of biological tissue for cardiac valve prostheses.

Cardiac valve prostheses provided with biological tissue valve flaps arecurrently utilised in clinical practice.

A prosthesis of this type is described, for example, in U.S. Pat. No.4,084,268.

Cardiac valve prostheses including valve flaps of biological tissueexhibit a more restricted thrombogenic activity and reproduce thefluodynamic process which characterises the operation of natural cardiacvalves with greater fidelity than cardiac prostheses comprising a rigidframe within which are mounted one or more shutters oscillating underthe thrust of the flow of blood.

To form such prostheses it has been proposed to fit a natural cardiacvalve, for example a pig's aortic valve, onto a support frame. Thissolution is, however, inconvenient to put into practice, given thedifficulty of easily obtaining a good natural starting material.

Another solution, which is very much more advantageous and functional,is to make the valve flaps of the prosthesis starting from biologicaltissue such as animal pericardium, preferably taken from cattle or pigs.The use of membranes of fascia lata or of dura mater of heterologous,homologous or autologous origin has also been proposed.

After removal, the tissue intended to be used for making the valve flapsis subjected to a cleaning operation and to a direct selection to retainonly those parts of it which are structurally most suitable.

The biological tissue is subsequently subjected to a treatment, called"fixation", performed, for example, by immersion in glutaraldehyde at acontrolled pH, possibly enriched with anticalcifying additives. Thefixation treatment (or "stabilisation" is another currently used term)is intended to encourage the establishment of cross links between thevarious forms of glutaraldehyde and the amino groups of proteinsconstituting the collagen of the tissue.

After the final fixation of the biological tissue, a cycle of cuttingand shaping operations permits the required shape to be imparted to thevalve flaps for mounting in the frame of the prosthesis.

After mounting in the frame of the prosthesis, during conservationbefore implanting in the patient, biological tissue valve flaps arenormally "held in shape", for example by means of cotton wads introducedbetween the frame and the valve flaps. Biological tissue which iscompletely fixed before shaping, in fact tends to revert to its naturalflat conformation.

Patent specification GB-A No. 2 046 165 addresses the problem ofobtaining a good fit between the biological tissue which forms the valveflaps and the frame of a cardiac prosthesis both in the case in which anatural valve, such as a pigs' aortic valve, is mounted in the frame andin the case in which the valve flaps are made starting from sheets ofbiological tissue such as pericardium. For this purpose it is proposedthat the final fixation of the biological tissue take place aftermounting it in the frame.

In the case in which the valve flaps are made starting from sheets ofbiological tissue, the incompletely fixed tissue is cut and applied tothe frame in such a way as to form three approximately shaped flaps. Thesupporting frame with the biological tissue flaps applied thereto isthen clamped between two complementary dies intended to impart the finalshape to the valve flaps. The assembly thus obtained is then immersed ina liquid based on glutaraldehyde which effects the final fixation of thetissue of the valve flaps whilst these are subjected to the action ofthe shaping dies.

This solution makes it possible to overcome the typical disadvantage ofprostheses the valve flaps of which are made from sheets of biologicaltissue completely fixed before the shaping of the flaps and theirmounting in the frame. As previously indicated, in such prostheses, thevalve flaps naturally tend to re-sume their natural flat conformation.

However, the process described in specification GB-A No.-2046165 doesnot permit the resolution of a particularly sensitive problem, that isto say that of stress which is imparted to the biological tissue sheetsupon mounting in the frame of the prosthesis and upon shaping of thevalve flaps. The biological tissue is in fact mounted in the frame,typically by sewing, before the shaping of the flaps is effected. Theperformance of the shaping operation involves, among other things, theapplication of deformation forces to the parts of the biological tissueconnected to the frame. The tissue can therefore be weakened in theseparts with the consequent risk of rupture of the prosthesis or, atleast, a significant reduction in the useful life of the prosthesis.

The shaping of the valve flaps by mechanical stamping means furtherinvolves, intrinsically, the application of stresses to the valve flaps.

The same specification GB-A No.-2 046 165 also describes a process forobtaining a cardiac prosthesis constituted by a frame in which there ismounted a pig's aortic valve. In this case the biological tissue isfixed by immersing the prosthesis in a container filled with fixationliquid. In particular, the prosthesis is applied to close the lower endof a vertical tubular duct in which a column of fixation liquid ismaintained, sufficient to ensure that between the two sides of theprosthesis a certain pressure difference is established. This pressuredifference encourages the establishment of a precise fit between theframe and the natural valve.

In this case, however, no shaping action of the valve flaps occurs sincethe flaps of the valves already have the necessary arcuate or bowl shapeconformation. Moreover the technique of fixation under a liquid pressuregradient described in specification GB-A No. 2 046 165 is not directlyapplicable to prostheses in which the valve flaps are made from sheetsof biological tissue such as pericardium. In this case, in fact, theparts of the tissue defining the valve flaps of the prosthesis do nothave, before shaping, the necessary matching shape to ensure aliquid-tight seal between the sides of the prosthesis.

The object of the present invention is to provide a process for makingvalve flaps of biological tissue for cardiac valve prostheses capable ofbeing put into practice economically on an industrial scale, overcomingthe disadvantages of the prior art processes described above.

The object is achieved, according to the present invention, by a processfor making, from sheets of biological tissue, valve flaps for cardiacvalve prostheses in which the said flaps are mounted in a support frameand in which sheets of incompletely fixed biological tissue, includingportions which can define the said valve flaps, are subjected to finalfixation whilst the said portions are maintained in a conformationsubstantially identical to the conformation of the valve flaps whenmounted in the prosthesis, characterised by the fact that it comprisesthe operations of:

providing forming means for the said sheets of biological tissue,capable of separating from each other in a substantially fluid-tightmanner the opposite faces of said valve flap defining portions

applying the said sheets of biological tissue to the said forming means,

generating a fluid pressure difference between the opposite separatedfaces of the said portions to produce the deformation of these portionstowards the said conformation substantially identical to theconformation of the valve flaps when mounted in the prosthesis,

effecting the final fixation of the biological tissue, and

separating the said sheets of biological tissue from the said formingmeans for subsequent mounting in the support frame.

The process of the invention allows valve flaps of biological tissue forcardiac prostheses to be made economically with characteristics of highreliability and durability.

The deformed shape in which the biological tissue of each flap isfinally fixed is, in fact, that in which the flap itself is mounted inthe prosthesis. In other words, since the fixation is effected in adeformed shape the valve flap stably assumes this deformed shape andtends spontaneously to return to this latter even after having beenstretched from this shape under the action of the flow of blood.

The process of the invention therefore allows a precise shaping of thevalve flaps before assembly in the prosthesis.

It is moreover possible to perform the shaping of the valve flaps whilstthe flaps themselves are subjected to a pressure range whichsubstantially reproduces the pressure range to which the flaps aresubjected in use. In particular, in prostheses provided with severalvalve flaps it is possible to impart to the flaps themselves, upon finalfixation of the biological tissue, a mutually matching configurationwhich can be exactly reproduced in the conditions of use.

With the process according to the invention, then, the risk ofdeformation stresses being imparted to the biological tissue iscompletely eliminated, particularly in the region of connection to theframe. The biological tissue is in fact mounted (sewn) to the frameafter having been finally fixed and shaped in the final conformation ofuse. With shaping under fluid pressure the intrinsic disadvantages ofmechanical shaping by stamping are also avoided.

The invention relates in particular to a process for making valve flapsfor cardiac prostheses comprising a frame capable of being traversed bya flow of blood and a sleeve of biological tissue with a plurality ofvalve flaps anchored to the frame along respective crescent shape edgesand provided with free edges, projecting inwardly of the frame, and ableto be separated by blood flowing through the prosthesis in one directionand to prevent the flow of blood in the opposite direction by moving toclosely matching positions under the pressure exerted by the blooditself, characterised by the fact that it comprises, in order, theoperations of:

providing a sleeve-forming element of substantially tubular form thewall of which has angularly adjacent apertures corresponding in numberto the number of valve flaps of the sleeve and separated from oneanother by shaped wall elements extending in an axial direction withrespect to the forming element; the said apertures having correspondingend edges the shape of which reproduces the shape of the said crescentshape edges of the valve flaps of the sleeve,

sealingly fitting a tubular sheath of incompletely fixed biologicaltissue onto the said forming element,

establishing between the interior of the forming element and theexterior of the said element a pressure difference such that theportions of the sheath of biological tissue facing the said apertures ofthe forming element are pressed inwardly of the said cavity in anarrangement in which the portions of the sheath deformed by the effectof such pressure difference converge radially inwardly into the formingelement in partially matching condition and each has at least onecrescent shaped edge,

effecting the final fixation of the biological tissue of the sheathwhilst maintaining the said pressure difference between the interiorcavity of the forming element and the exterior of the element itselfsuch that each of the said portions assumes, in a substantially stablemanner, by virtue of such final fixation, the deformed conformationachieved by the effect of the said pressure difference, and

separating the finally fixed biological tissue of the sheath along aline joining the ends of the crescent shaped edges of the said portionsof the stably deformed sheath to create, in each of such portions, afree edge defining the free edge of one of the valve flaps of thesleeve.

According to another aspect, the present invention provides apparatusfor the production of valve flaps for cardiac prostheses comprising aframe which can be traversed by a flow of blood and a sleeve ofbiological tissue with a plurality of valve flaps anchored to the framealong respective crescent shaped edges and provided with free edges,projecting inwardly of the frame, able to be separated by blood flowingthrough the prosthesis in one direction and to prevent the flow of bloodin the opposite direction by moving to closely matching positions underthe pressure exerted by the blood itself, characterised by the fact thatit comprises,

a reservoir which can be filled with a fluid for the fixation of thebiological tissue,

at least one forming element for the said sleeve, projecting into thesaid reservoir in a position immersed in the said fixation liquid, thesaid forming element being of substantially tubular form with aperipheral wall traversed by adjacent apertures, equal in number to thenumber of valve flaps of the sleeve, separated from one another byshaped wall elements extending in an axial direction with respect to theforming element; the said apertures having corresponding edges the shapeof which reproduces the shape of the crescent shaped edges of the valveflaps of the sleeve, and

means for establishing a pressure difference between the fixation fluidwithin the reservoir and the interior cavity of the said formingelement.

The invention will now be described, purely by way of non limitativeexample, with reference to the attached drawings, in which:

FIG. 1 is a perspective view of a cardiac valve prosthesis provided withvalve flaps formed according to the invention,

FIG. 2 is an axial view from above of the valve prosthesis of FIG. 1,

FIG. 3 is a section taken on the line III--III of FIG. 2;

FIG. 4 is a perspective view of one of the elements of the prosthesis ofFIGS. 1 to 3;

FIG. 5 illustrates an intermediate stage in the production of theelement of FIG. 4;

FIG. 6 schematically illustrates apparatus for the performance of theinvention;

FIG. 7 is a perspective view on an enlarged scale of one of the elementsof the apparatus of FIG. 6;

FIG. 8 is a section taken on the line VIII--VIII of FIG. 7, and

FIGS. 9 to 12 schematically illustrate various successive stages in theprocess of the invention.

In FIGS. 1 to 3 a cardiac valve prosthesis is generally indicated by thereference numeral 1; this is intended to be implanted in a cardiac wallto replace a natural valve.

In the implantation position the prosthesis is sutured to the cardiacwall in the zone surrounding the orifice formed by removal of theautologous valve membranes. Structurally, the prosthesis is constitutedby a support structure (frame) of generally annular form, which isintended to be sutured to the cardiac wall and to receive within it avalve sleeve including valve flaps of biological tissue. As is known tothe expert in the art and as will be better illustrated below, theprosthesis is intended to be traversed by a flow of blood in thedirection schematically indicated by the arrow in FIG. 1 and to preventthe flow of blood in the opposite direction.

The frame of the prosthesis, generally indicated 2, includes a rigid orsemi-rigid stent having a set of three shaped projections 4 (FIG. 3).

The stent 3 and the projections 4 are normally constituted by a singlepiece of biocompatable material such as, for example, titanium, achrome-cobalt alloy or one based on cobalt, or else the plasticsmaterials known by the commercial names "Teflon" or "Delrin".

The stent 3 and the projections 4 are encased in a biocompatible textile5 such as, for example, a textile made with the yarn known by thecommercial name "Dacron".

The textile 5 forms, on the outer face of the stent 3, a wide annularloop 6 constituting a ring for the suture of the prosthesis to thecardiac tissue.

Within the loop 6 there is normally provided an annular pad 7 ofbiocompatable material, constituting a reinforcing core for the suturering of the prosthesis. The pad 7 is constituted by a ring of fabricwhich can easily be traversed by the surgical thread utilised for thesuture of the prosthesis to the cardiac tissue.

The textile 5 is wound around the stent 3 and subsequently closed in agenerally tubular configuration by suture stitches indicated 8.

In the embodiment of FIGS. 1 to 3, suture stitches 8 are disposed incorrespondence with the terminal edge of the stent 3 from which theprojections 4 extend and in correspondence with the region of connectionof the suture ring 6,7. Other arrangement for achieving the same finalresult are naturally possible.

To the textile 5, and possibly also on the thread constituting thesuture stitches 8, there is applied (before or after mounting on thestent 3) a coating of biocompatable carbonaceous material 5aconstituted, for example, by graphite, glassy carbon or carbon having aturbostratic structure.

The coating 5, which significantly improves the anti-thrombogenicproperties of the textile 5, is applied by cathodic spraying(sputtering) utilising a target constituted by a carbonaceous material,normally selected from the group comprising graphite, glassy carbon andcarbon with a turbostratic structure. The application by cathodicspraying is described in a detailed manner in U.S. patent application,Ser. No. 545,292, filed Oct. 25, 1983, now abandoned in favor of U. S.continuation patent application, Ser. No. 799,561, filed Nov. 20, 1985,by the same applicant, the description of which is incorporated hereinby reference. The application of the coating 5a by cathodic spraying canbe effected at a temperature close to ambient temperature, avoidingdamage to the textile 5 or the material of the stent 3 in the case inwhich the coating 5 is applied after the textile 5 has been fixed to thecore 3.

The interior part of the prosthesis 1 is occupied by a valve sleeve 9 ofbiological tissue including three valve flaps indicated 10.

The sleeve 9 is made of an inert biological material. Biological tissueswhich have been used with success are cow or pig pericardium tissues,although the use of biological tissues of other nature and origin is notexcluded. For example, it has been proposed to utilise as biologicaltissue a membrane of cranial or cervical dura mater taken from animals,or even a membrane of human or animal fascia lata.

After removal, the biological tissue is subjected to a cleaningoperation. Subsequently there is effected a selection of the tissue withthe intention that only the structurally most homogeneous and suitableparts of it are to be retained.

The selected layers of biological tissue are then subjected to atreatment operation intended to stabilise the helastic and mechanicalstrength thereof and to confer on them characteristics of chemicalinertness with respect to blood.

These operations, generally known as "fixation" or "stabilisation"operations, are normally performed by immersing the tissue in solutionsof glutaraldehydes with controlled pH, possibly enriched withanticalcifying additives.

The fixation operation generally involves the formation of stable crosslinks between the various forms of the glutaraldehyde and the aminicgroups of the proteins constituting the collagen of the tissue.

The treatment times can vary widely in dependence on the characteristicsof the biological tissue subjected to the fixation and the manner inwhich the fixation operation is performed. During the course of thetreatment process, the concentration of the fixation solution is varied.For example, in the case in which solutions of glutaraldehyde are used,an initial phase, the said prefixation, is performed with a solution ofglutaraldehyde in a concentration of the order of 0.2% which increasesto a final fixation phase in which the concentrations are of the orderof 0.5%.

For the purpose of understanding the invention it is necessary todistinguish between an incompletely fixed biological tissue (that is tosay, a tissue subjected only to prefixation) and a completely fixedfixed biological tissue. The incompletely fixed tissue in fact retainscharacteristics of plastic deformability which allow shaping operationsto be performed thereon. The finally fixed tissue on the other hand hasdifferent elastic characteristics such that, after a possibledeformation, the tissue tends to return spontaneously to theconformation assumed upon fixation.

As can be seen in FIGS. 4 and 5 which illustrate the sleeve 9 in theassembled configuration of the prosthesis and in open development, thesleeve 9 is constituted by two layers of biological tissue one of which(inner layer) constitutes the sleeve proper and is provided with shapedparts constituting valve flaps 10. The other layer of biological tissue(outer layer), indicated 9a, constitutes a tubular support covering forfixing the sleeve to the frame 2. For this purpose, in correspondencewith the valve flaps 10 the layer 9a has crescent shape notches 9b theshape of which reproduce in development the shape of the sides of theprojections 4 of the stent 3 of the prosthesis frame.

The two biological tissue layers constituting the sleeve 9 are suturedtogether with surgical thread along suture lines, preferably of thezig-zag type, which extend along crescent shape paths and each of whichdefines a crescent shape edge 10a of a respective valve flap 10.Preferably the thread utilised for the suture lines 10a is provided witha coating of biocompatible carbon material as described with referenceto the textile 5.

In a manner which will be described in more detail below the valve flaps10 have imparted to them a general bowl-shape configuration theconcavity of which faces the layer 9a.

Consequently, when the two layers of biological tissue sutured togetherare wound into a tube by suturing together two opposite edges of thelayers along a line of stitching indicated 11, the free edges of thevalve flaps 10, indicated 10b, converge towards the interior of thesleeve, being arranged in a closely matching star shape configurationwhich can be seen in FIG. 2.

As can be seen in FIGS. 4 and 5 the sleeve has a generallyfrusto-conical configuration which, although not essential, has beenfound to be preferable for the functional purposes of the prosthesis.

The mounting of the sleeve 9 on the frame 2 is normally effected bysuturing the layer 9a onto the cladding textile 5 along the end edges ofthe frame 3 and the projections 4 as is schematically illustrated inFIG. 3.

On the opposite side of the free edges 10b of the valve flaps 10 theinner layer of the sleeve is provided with a terminal portion 12 whichextends beyond the corresponding end edge of the layer 9a and can beturned inwardly of the frame 2 and be sutured to the textile 5 adjacentthe inner edge of the suture ring 6,7.

The conformation of the sleeve 9 and its disposition upon assemblywithin the frame 2 are such that substantially the whole of the surfaceof the prosthesis intended to be invested with the blood flow is coveredwith biological material having significant antithrombogenic properties.

Making reference, by way of example, to an atrioventricular implantationarrangement, in the diastolic phase the blood which flows out of theatrium enters the ventricle and traverses the prosthesis in thedirection schematically indicated by the arrow in FIG. 1. In thisdirection of flow the blood flows over the convex face of the valveflaps 10, separating their free edges 10b and forming a substantiallycylindrical central aperture in the prosthesis body, through which theblood can flow freely.

As soon as a pressure difference sufficient to cause the blood to flowin the opposite direction is established across the prosthesis by theeffect of the contraction of the ventricle, the pressure exerted by theblood itself on the concave faces of the valve flaps 10 forces the freeedges 10b into he closely matching position illustrated in FIG. 2. Inthese conditions the blood flow across the prosthesis is prevented.

In FIG. 6 there is generally indicated with the reference numeral 20apparatus which can be used for shaping the valve flaps 10 of thebiological tissue sleeve indicated by the reference numeral 9 in thepreceding Figures.

The apparatus 20 includes a reservoir 21 intended to receive a solutionL for the fixation of the biological tissue. The reservoir 21, (which isillustrated in median vertical section) has a generally drum-shapeconfiguration and is constituted by a tubular peripheral wall 21a theopenings at the ends of which are closed by a cover 21b and by a bottomwall 21c, constituted by plate elements of circular form. Between thecover 21b and the bottom wall 21c are interposed tie elements 21d whichtightly hold the cover 21b and the bottom wall 21c onto the peripheralwall 21a ensuring fluid-pressure tight sealing of the reservoir 21.

The fixing solution L is taken from a collection reservoir 22 andconveyed into the reservoir 21 by means of a pump 23 through a duct 23aprovided in the bottom wall of the reservoir 21. Between the pump 23 andthe duct 23a there is interposed a valve 24 intended to prevent thereturn of the solution L towards the collection reservoir 22 when, as isdescribed in greater detail below, the solution contained in thereservoir 21 is put under pressure.

The fixation solution L introduced into the reservoir 21 is in general asolution intended to perform the final fixation (terminal fixation) of abiological tissue, for example a 0.5% solution of glutaraldehyde.

In general, the reservoir 21 is not completely filled with the solutionL. Above the free surface of the solution L there is thus defined achamber 25 into which a gas under pressure derived from a sourceconstituted, for example, by a gas bottle 26, can be admitted through anaperture 25a provided in the side wall of the reservoir 21. In theconnection pipe between the source 26 and the chamber 25 there isinterposed a pressure regulator 27 which allows regulation of the gaspressure in the chamber 25, and, consequently, of the hydrostaticpressure of the solution L within the reservoir 21.

In the cover 21b of the reservoir 21 there are provided threadedapertures 28, each of which constitutes a seat for mounting a formingelement 29, one of which is illustrated in greater detail in FIG. 7.

In the cover 21b there is normally provided a plurality of apertures 28,only one of which is visible in FIG. 6, which represents a section ofthe reservoir 21 taken on a diametral plane of the reservoir itself. Theapertures 28 are distributed around a circular track concentric with theperipheral wall 21a of the reservoir. Each communicates through arespective radial duct 28a extending through the cover 21b with acollection cavity 128 formed in a central position in the wall of thecover 21b. The cavity 128 communicates with the suction side of a pump129 the delivery side of which is connected to a breather duct whichopens into the interior of the collection reservoir 22.

Each forming element 29 is substantially constituted by a frusto-conicalbody 30 having a tubular structure, supported at its larger base by asleeve body 31 externally threaded at 31a. The inner cavity of thesleeve body 31 communicates with the inner cavity of the frusto-conicalbody 30. In the assembly disposition of the elements 29 in the reservoir21 the sleeve body 31 of each element 29 is screwed into the associatedaperture 28 in such a way that the tubular body 30 supported by itprojects into the interior of the reservoir 21 so as to be substantiallyimmersed in the fixing solution L when the reservoir 21 is filled.

At the end facing outwardly of the reservoir 21 each aperture 28 isclosed by an insert 32 of transparent material (for example plexiglass)which allows the interior of the frusto-conical body 30 of the formingelement 29 screwed into the aperture 28 to be observed from the outside.

The tubular body 30 of each forming element 29 has an intermediate bodyportion with three apertures 34 angularly adjacent one another andseparated by shaped wall elements 35 extending axially with respect tothe body 30 itself. Each element 35 has a generally flattened form inthe radial direction with respect to the body 30, with a biconvexsymmetrical shape. On the side facing outwardly of the body 30, eachelement 35 is delimited, for reasons which will be illustrated betterbelow, by a rounded surface 35a free from sharp corners or otherdiscontinuities.

At the end facing the sleeve body 31 each aperture 34 has a terminaledge 34a the shape of which reproduces the shape of the crescent-shapeedges 10a of the valve flaps 10.

On the outer surface of the tubular body 30, above and below theapertures 34 respectively, there are provided annular grooves 36 thefunction of which will be illustrated below.

The apertures 34 and the grooves 36 are normally formed by mechanicalworking of the forming element 29, which is constituted by a singlepiece of plastics material such as the materials known by the commercialnames "Teflon" or "Delrin".

The diametral dimensions of the frusto-conical body 30 of each formingelement 29 are substantially identical with the diametral dimensions ofthe sleeves 9 which it is intended to make.

In use of the apparatus, sheets of incompletely fixed biological tissue,(that is to say sheets of biological tissue subjected only to theprefixation operation) are formed into a tube by suturing together twoopposite edges of the sheet itself so as to form tubular sheets 90 offrusto-conical form which can be fitted over the bodies 30 of theelements 29 as is schematically illustrated in FIG. 9.

In both FIG. 9 and FIG. 10, only the portion of the forming element 29comprising the body portion with the apertures 34 is illustrated. Thedimensions of the sheath 90 are chosen in such. a way that each sheathforms, with respect to the corresponding forming body 30, a loosecoupling.

After having been fitted onto the forming body 30 each sheath 90 issecurely fixed onto the forming element, for example by means of tworesilient seals 37, of the type usually called "O rings" which engagethe grooves 36. The suture line along which the sheath 90 has beenclosed into a tube is positioned in correspondence with one of the wallelements 35.

The sheath 90 is thus fitted with a fluid tight seal onto the associatedbody 30 in an arrangement in which the sheath portions extending acrossthe apertures 34 constitute diaphragms which separate the internalcavities of the tubular body 30 from the exterior of the forming element29.

Normally, the sheaths are mounted on the forming bodies 30 with theelements 29 fixed to the cover 21b of the reservoir 21 remote from thereservoir itself.

After having fitted the sheaths 90 onto the elements 29 and beforefinally locking the cover 21b onto the reservoir body 21, the pump 129can now be activated for a short time in such a way as to create avacuum within the cavities of the forming elements 29. Under the actionof this vacuum the sheath portions 29 extending through the apertures 34are, so to speak, "sucked" into the interior of the forming bodies. Thedeformed conformation thus assumed by such sheath portions can be seenthrough the transparent insert 32. It is thus immediately possible todetect the presence of defects (for example non-uniformity) and errorsin mounting the sheaths 90 in such a way as to be able to replacedefective sheaths and eliminate such mounting errors before proceedingto the shaping and fixation treatment of the biological tissue.

To effect such treatment the cover 21b carrying the elements 28 on whichthe sheaths 90 are sealingly fitted is closed over the reservoir 21 inthe arrangement schematically illustrated in FIG. 6. The pump 23 is nowactivated making the fixation solution L flow into the interior of thereservoir 21. The level of the solution L is regulated in such a waythat the whole of the sheath 90 is immersed in the fixation solution.Preferably a small quantity of solution L is also introduced into theinterior of the forming bodies 30 in such a way as to act on the innersurface of the sheath 90.

After having closed and sealed the reservoir 21 the supply source 26 andthe pressure regulator 27 are activated in such a way as to establish acontrolled pressure within the solution L.

The pump 129 remains inoperative so that the inner cavity of eachforming body 30 is practically at atmospheric pressure. Consequently thepressurisation of the solution L within the reservoir 21 is such that apressure differential is established across the apertures 34, whichcauses deformation of the portions of the sheath 90 covering theapertures 34. The fixation solution acts on such sheath portions todilate them and press them into the tubular body 30 in a disposition inwhich, as is schematically illustrated in FIG. 11, the median parts ofsuch portions are positioned in mutual contact with a star-shapegeometry substantially similar to that illustrated in FIG. 2 withreference to the valve flaps 10.

Naturally, the resistance afforded by the tissue of the sheath 90 to thepressure exerted by the fixation solution varies in dependence on thenature of the biological tissue, on its thickness and the dimensions ofthe apertures 34. The gas pressure within the interior of the chamber25, which determines the pressure of the solution L, is regulated insuch a way as to bring the deformed portions of the sheath to aconfiguration of mutual matched shaping substantially similar to that ofthe valve flaps 10 of the sleeve 9.

The instantaneous configuration reached by the deformed portion of thesheath 90 can be observed by an operator through the transparent inserts32. It is therefore possible gradually to increase the pressure of thefixation solution until the configuration of mutual shape matching ispositively achieved.

Each deformed portion of the sheath 90 then has a general bowl-shapeconfiguration and is delimited on one side by a crescent-shape edge theshape of which reproduces the shape of the terminal edge 34a of theaperture 34 and, consequently, the crescent-shape edge 10a of one of thevalve flaps 10. In other words, in each of the portions there is formeda shaped element of stably fixed biological tissue the conformation ofwhich is exactly similar to the conformation of one of the valve flaps10 of the sleeve 9.

The pressure difference which produces the deformation of the sheath 90is maintained for the period necessary to produce complete fixation ofthe biological tissue of the sheath 90 by the solution L.

According to the invention the biological tissue intended to constitutethe sleeve 9, and in particular to valve flap 10, is subjected to ashaping operation which makes it assume the final conformation of usewhen the tissue is still not completely fixed. The final or completefixation is effected when the biological tissue has already beendeformed making it assume the final conformation of use.

In this way the fixed biological tissue tends to reassume, after anyaccidental deformation, the conformation in which the tissue was mountedin the prosthesis in the form of a valve flap.

Moreover, the existence of a pressure gradient across the deformedportions of the sheath 90 encourages the diffusion of the fixationsolution L across the biological tissue ensuring an intimate penetrationthereof into the tissue. This also allows the treatment times necessaryto obtain final fixation of the tissue to be significantly reduced.

The effect of the pressure gradient which is established across thesedeformed portions is that the solution L in fact seeps through thebiological tissue, penetrating into the tubular body 30 of the formingelement 29.

The duration of the fixation operation can be chosen in dependence onthe pressure at which the solution L is delivered (that is to say, independence on the pressure gradient applied across the two faces of eachportion of the sheath 90 extending across one of the apertures 34) insuch a way that the deformed portions of the sheath 90 are intimatelypermeated by the solution L.

Further, the fact that the portions of the sheath 90 intended toconstitute the valve flaps 10 of the sleeve 9 assume their finalconformation under the action of a fluid under pressure permits theshaping of such flaps whilst the flaps themselves are subjected to apressure range which substantially reproduces the pressure range towhich the flaps are subjected in use. In this way, upon final fixationof the biological tissue, there is obtained a mutual shape-matchingconfiguration between the flaps which can be exactly reproduced in theconditions of use. This also avoids the possibility of non-uniformstresses and strains arising in such portions which could prejudice thecorrect operation of the prosthetic valve flaps. The conformation of thewall elements 35 and, in particular, the presence of the roundedsurfaces 35a on the radially outer side of each element avoids thepossibility of stress phenomena or lesions arising during the shapingand final fixation operation in the regions of biological tissuestretched out over the elements 35, such as would prejudice the strengthof the tissue.

In the preceding part of the description, explanation has been givenwith reference to a situation of use of the apparatus 20 in which thepressure gradient applied between the opposite faces of each of theportions of biological tissue defining the valve flaps 10 is exclusivelyderived from the pressure applied to the fixation solution L within thereservoir 21. It is, however also possible to establish the saidpressure gradient by the effect of a combined action of the pressureapplied to the fixation solution L and the vacuum created within thecavity to each shaping element 29 by the pump 129. In this case the pump129, which allows (as previously described) a preliminary control of thestructural characteristics and the exact positioning of the sheaths 90mounted on the shaping elements 29, is also activated during the finalfixation operation on the biological tissue, by jointly adjusting theeffect of pressurisation of the solution L by the gas taken from thesource 26 and the degree of vacuum generated within the shaping elements29 by the pump 129.

It is also possible to envisage the use of apparatus 20 in which thepressurisation system formed by the gas source 26 and the regulator 27is eliminated. In this case the pump 129 is activated both to perform apreliminary check on the structural characteristics and exactpositioning of the sheaths 90 mounted on the shaping elements 29, and togenerate, after the reservoir 21 has been filled with the fixationsolution L, the pressure gradient which determines the deformation ofthe portions of biological tissue extending across the apertures 34.

In structural terms, the said gradient can be established in at leastthree different ways, that is to say:

(i) by applying (for example by means of gas taken from the source 26) apressure to the fixation solution L within the reservoir 21, maintainingthe internal cavities of the forming elements 29 substantially atatmospheric pressure,

(ii) by applying the said pressure to the fixation solution L andsimultaneously creating (for example by operation of the pump 129) avacuum (a pressure less than atmospheric pressure) in the interiorcavity of the forming elements 29, and

(iii) exclusively by the effect of the vacuum created in the interiorcavities of the forming elements 29, whilst the solution L is maintainedat substantially atmospheric pressure.

Upon completion of the fixation operation, the source which caused thesaid pressure gradient are de-activated and the cover 21a is removedfrom the reservoir 21. The sheaths 90 can then be disengaged from theforming elements 29 by removing the sealing rings 27.

After the removal of the stitches previously applied to effect theclosure into tubular form, the sheet constituting each sheath 90 isagain opened out, assuming the conformation schematically illustrated inFIG. 12, in which the sheet of biological material, initially flat, nowhas three bowl-shaped parts substantially equal to one another anddelimited on corresponding sides by crescent shape edges constitutingthe crescent edges 10a.

In other words, the sheet of biological tissue obtained starting fromthe sheath 90 subjected to the final fixation treatment incorporates avalve sleeve provided with three completely formed and shaped flaps 10.

The separation of the biological tissue of the sheath 90 along a line 91which joins the ends of the crescent shaped edges 10a permits theseparation of the frustoconical sleeve from the remaining part of thesheath 90 intended to be discarded, forming the free edges 10b of thevalve flaps 10.

In the illustrated embodiment the separation of the biological tissue ofthe sheath 90 is effected after the sheath 90 has been released from theforming element 29 and returned to an open position. It is, however,possible to effect separation of the sheath 90 when it is still closedin the form of a tube, possibly when it is still fitted onto the formingelement 29.

The valve sleeve 9 is subsequently mounted on the prosthesis accordingto the criteria described in the introductory portion of the presentdescription.

What is claimed is:
 1. A process for the manufacture of valve flaps fora cardiac prosthesis from a sheet of biological tissue free of suchflaps, comprising:providing forming means having spaced aperturestherein, applying the sheet of biological tissue free of valve flaps tosaid forming means with portions of the biological tissue extending overthe spaced apertures in a fluid-tight manner, generating a fluidpressure differential across opposite surfaces of said portions of thebiological tissue extending over said apertures to deform said portionsinto conformations substantially the same as the conformations of thevalve flaps when mounted in a prothesis, effecting formal fixation ofthe biological tissue while maintaining a pressure differential acrosssaid opposite surfaces of said formed valve flap portions tosubstantially stablize said valve flap portions, and removing thebiological tissue with its substantially stable valve flap portions fromsaid forming means.
 2. The process defined in claim 1, wherein the finalfixation of the biological tissue is effected by means of a fixationfluid brought into contact with at least one of the opposite faces ofthe said portions of biological tissue.
 3. The process defined in claim2, wherein said pressure difference is generated by pressurising thefixation fluid and bringing the pressurised fixation fluid into contactwith one of the opposite faces of the said portions of biologicaltissue.
 4. The process defined in claim 3, wherein said portions ofbiological tissue are substantially immersed in the fixation fluid andwherein the said pressure difference is established between the bodiesof fixation fluid which are in contact with the opposite faces of thesaid portions.
 5. The process defined in claim 4, wherein the magnitudeof said pressure difference and the length of time for fixation of thebiological tissue are selected so as to effect a substantial diffusionof the fixation liquid through the said deformed portion of biologicaltissue.
 6. A process for the manufacture of valve flaps for cardiacprostheses of the kind comprising a frame capable of being traversed bya flow of blood and a sleeve of biological tissue, with a plurality ofvalve flaps anchored to the frame along respective crescent shaped edgesand provided with free edges projecting inwardly of the frame, capableof being separated by blood flowing through the prosthesis in onedirection and of preventing the flow of blood in the opposite directionby moving to closely matching positions under the pressure exerted bythe blood itself, wherein the process comprises:providing a formingelement of substantially tubular form for the sleeve, the wall of whichhas angularly adjacent apertures corresponding in number to the numberof valve flaps of the sleeve and separated from one another by shapedwall elements extending in an axial direction with respect to theforming element; said apertures having corresponding end edges the shapeof which reproduces the shape of the said crescent shaped edges of thevalve flaps of the sleeve, sealingly fitting a tubular sheath ofincompletely fixed biological tissue over said forming element,establishing a pressure difference between the interior and the exteriorof the forming element such that the portions of the sheath ofbiological tissue facing the said apertures of the forming element arepressed inwardly of said cavity in an arrangement in which the sheathportions deformed by the effect of such pressure difference convergeradially into the forming element in partial matching conditions, eachhaving at least one crescent shaped edge, effecting the final fixationof the biological tissue of the sheath while maintaining said pressuredifference between the interior cavity of the forming element and theexterior of the element itself, such that each of the said portionsassumes, in a substantially stable manner, the deformed conformationachieved by the effect of the said pressure difference, by virtue ofsaid final fixation, and separating the finally fixed biological tissueof the sheath along a line joining the ends of the crescent shaped edgesof the said stably deformed portions of sheath to create in each of suchportions a free edge defining the free edge of the valve flap of thesleeve.
 7. The process defined in claim 6, wherein the said sheath isobtained starting from a sheet of biological tissue formed into a tubeby bringing two opposite edges of such sheet together and connectingthem by suturing said edges.
 8. The process defined in claim 6 whereinthe tubular sheath is fixed to the forming element by means of resilientsealing rings fitted over the forming element on opposite sides of saidapertures.